What is the effect of placing a bypass capacitor across RE on AC gain?

• A. Applying a bypass capacitor across RE reduces emitter degeneration at signal frequencies, increasing gain toward -RC / r_e.
• B. Bypass capacitor across RE reduces gain.
• C. Bypass capacitor across RE has no effect on AC gain.
• D. Bypass capacitor across RE only affects input impedance.

Answer: (A)

The idea being tested is how emitter degeneration shapes AC gain and what a bypass capacitor does for that degeneration. When an emitter resistor is in the circuit, the emitter current flowing through RE creates a voltage that feeds back into the base-emitter junction. This negative feedback reduces the transistor’s effective transconductance and lowers the small-signal gain. In a simple view, the AC voltage gain of a common-emitter stage with emitter degeneration is roughly -RC divided by (re plus RE), so increasing RE lowers the gain.

Placing a capacitor across RE for AC signals provides a low-impedance path around RE. At the signal frequencies where the capacitor looks like a short, the emitter is effectively at AC ground, so the emitter degeneration is removed from the AC path. The emitter resistance that the signal “feels” becomes just the intrinsic re, and the gain climbs toward -RC/re, which is the maximum gain set by RC and the transistor’s intrinsic emitter resistance. The DC bias remains unchanged because the capacitor blocks DC, keeping the quiescent point intact. At very low frequencies, the capacitor isn’t a perfect short, so some degeneration persists and the gain is reduced; at mid to high frequencies, bypassing is effectively complete and the gain is maximized.